With widespread of portable electronic apparatuses and the like, lithium ion secondary batteries are regarded as important devices as power sources therefor. Further, with widespread of smartphones and wearable instruments in recent years, high safety, high life span, high energy density, and the like are required in the lithium ion secondary batteries. In addition, charging the lithium ion secondary batteries in a short time while degradation in capacity at the time of long-term use (cycle degradation) is suppressed becomes an important problem.
In this regard, as a charging method for a lithium ion secondary battery of the related art, there are used methods such as a constant current and constant voltage method (CC-CV method) in which charging is started at a constant current and charging is performed at the time point of reaching a predetermined voltage while a constant voltage is held; a multi-stage charging method in which charging is started at a constant current and charging is performed by lowering a current value stepwise at the time point of reaching a predetermined voltage; and a stepwise charging method in which charging is started at a constant current, supplying current is stopped at the time point of reaching a predetermined voltage, and then charging is intermittently repeated.
Further, for example, in a charging method disclosed in Japanese Patent Application Laid-Open No. 2011-024412, a lithium ion secondary battery is charged fast using a variable charge-profile.